An intriguing technological area which has been the subject of increasing interest is controlled environment agriculture (hereinafter referred to as CEA). In a natural environment, agriculture productivity is limited by the seasonal distribution of sunlight, temperature and moisture. In a CEA system the light energy required for growing can be provided from lamp sources geared towards maximized day lengths, intensity levels and photosynthetically useful light spectrum bands. Along with optimized temperatures, moisture and nutrients, the growing periods can be extended to long day light periods the year round with substantial increases in yields. The chief factor limiting CEA application has been the economics of such systems with previously available technology. Plant researchers have proven the potential of CEA and in recent years commercial CEA growing of seedling plants has been demonstrated to be practical. Inevitably, broader application of CEA will follow and it will have significant implications for land use, energy consumption and food quality, plus better controlled use of pesticides and fertilizers and other factors that affect the world we live in.
To aid in understanding the economics of CEA a brief review of the process of photosynthesis is helpful. It is also important to remember that the net growth potential of a plant is severely limited when growing in a natural environment and since farming began, man has quested to modify the natural environment of growing plant life to improve output. Thus, to begin the review, it is first noted that between 5 and 10% of a plant is derived from minerals and fertilizers taken up through the root system from the growing medium. It might be noted here that the worth of systems like "hydroponics" is often overemphasized since, in reality, such systems contribute to only a small fraction of a plant's growth, their actual importance lying in seeing to it that a plant is not limited as far as minerals and water is concerned. The remaining 90 to 95% of a plant is chemically synthesized by the process known as photosynthesis which, using light as the primary energy source, removes carbon dioxide (CO.sub.2) and water (H.sub.2 O) from the environment and returns carbohydrate compounds and oxygen to the environment.
Primary production of the carbohydrate compound glucose, from which most plants are built, takes place within the leaves of a plant in cells containing chlorophyll. The chlorophyll molecules trap light photons and use these packets of energy to drive a sequential series of chemical reactions. It should be remembered that the process is sequential. Specifically, glucose, a simple sugar, is initially produced and is then reprocessed to several different compounds, ending up as some final form of carbohydrate material. The chlorophyll is not consumed in the chemical reactions but its presence, plus that of a number of mineral salts and a suitable temperature environment, is essential to both the primary production of the glucose and subsequent processing. The reaction chain is much too complex for elaboration here and suffice it to say that the final carbohydrate compounds formed are composed of some multiple of the empirical formula CH.sub.2 O. The simplified equation for photosynthesis is H.sub.2 O + CO.sub.2 light CH.sub.2 O + O.sub.2.
The carbon products manufactured by the photosynthetic process are nominally stable in air at room temperature and can be considered to be stored energy. Further, when heated, these products burn and the reaction is reversed. In the reverse process, the carbohydrate and oxygen react thereby returning carbon dioxide, water and thermal energy back to the environment. Studies of this reverse reaction show that 112 KCAL of thermal energy would be evolved for each mole equivalent of CH.sub.2 O. If 112 KCAL are evolved in this reverse reaction the first law of thermodynamics dictates that at least that amount of light energy was utilized in a chemically useful way in order to manufacture the carbohydrate initially.
The chemical equation for the primary production of the photosynthate glucose when balanced with the 112 KCAL of energy known to be fixed in the process is as follows:
From the From the From a light Chemical To The Growing Atmosphere source (in the Store Atmosphere Medium forward direction) H.sub.2 O + CO.sub.2 112 KCAL Light Energy 1/6(C.sub.6 H.sub.12 O.sub.6) + O.sub.2 .revreaction. 122 KCAL Thermal Energy which can be converted to: 18 g 44 g .13 kWh Light Energy 30 g 32 g Water + Carbon .revreaction. Carbohydrate + Oxygen Dioxide .13 kWh Thermal Energy
A cursory examination of the relative costs of furnishing the required amount of light energy, water, carbon dioxide, minerals and heat shows that the light cost far exceeds the combined cost of all others. Likewise, only the carbon product output has a dollar value of interest. Accordingly, the present analysis will now be confined to CEA light costs and the output of interest. It should be noted that a plant also uses between 20 to 40% of the initially produced glucose for housekeeping purposes and that the remainder goes to plant growth product.
To proceed with the examination, a plant or crop suitable for CEA will be considered. Tomatoes are a good candidate since they can profitably utilize long day periods. Further, tomatoes are in demand the year around, with most major markets being heavily dependent on imports most of the year. The fresh weight of an average successful tomato plant is about 15 kilograms, with about 50% of this figure being the weight of the fruit. The dry weight of the fruit runs about 5% and the vine, roots, leaves and the like, about 10%. Hence, calculation shows the synthesized material to be about 1125 g. If this figure represents the final form, which is representative of 60% of the glucose manufactured, then about 1875 g of glucose is required. The light requirement of 0.13 kWh for 30 grams extended to 1874 g means that about 8 kWh of equivalent light energy has to be utilized in a chemically useful way in manufacturing the plant. With the amount of energy required to be fixed in the representative tomato plant converted to an energy term commonly used when discussing electrical energy, what amount of electrical input energy is required to fix the 8 kWh of light energy stored in the plant? To establish this value the pathway taken by the input energy is required and this is a function of the type of light the plant best utilizes vis a vis a selected light source and the light-time relationships involved.
Three important factors enter into the source selection. First, the light absorption curve for chlorophyll indicates that the efficiency at which chlorophyll electrons absorb light is dependent to a marked degree on the wavelength of the light photons. In particular, it has been found that a large majority of the light photons in the vicinity of 670 nanometers will be absorbed, perhaps half as many of the 450 nanometer photons will be absorbed, and a lower percentage in the green-yellow 500 to 600 nanometer spectral region will be absorbed. It is interesting to note that this is why plants look green in nature, i.e., many of the green photons are not absorbed but are reflected. It is obvious that the selection of a green emitting light source would not be very efficient if a large percentage of its photons would be reflected rather than used. On this basis, a lamp rich in 670 radiation would be most efficient and a blue light would be the next insofar as absorption considerations are concerned.
The second important factor is the energy utilization of each photon absorbed. Except in isolated instances it is known that an electron absorbing a photon on visible light energy is raised to either the first, second or third singlet state depending upon the wavelength of the photon. Since the wavelength is inversely proportional to the energy content, the shorter wavelength photon has more energy, i.e., a 450 nm blue photon has roughly 11/2 times the energy of a 680 nm red photon. An absorbed 450 nm photon would raise the electron to the third singlet state of excitation whereas a 680 nm photon would raise the electron to the first singlet state. This is very important because it is also known that only the energy from a first singlet state can proceed to be utilized in a chemically useful way. Any electron excited above one of lower substates of the first singlet state must reduce itself by vibrational or rotational processes to a lower first singlet substate. This energy reduction, which contributes only to the kinetic motion of the molecules and not to photosynthesis, is called internal energy conversion. Once an excited electron reaches the lower substate levels of the first excited singlet state it can then either be utilized in a chemically useful way or continue the internal conversion process or be fluoresced or go to a triplet state. The foregoing clearly indicates that for primary photosynthate production the use of a red light energy source would be the most efficient. It should be noted, however, that some shorter and longer wavelength energy may be required for morphological and other processes.
The third important factor is once the photon energy is reduced to the first singlet useable state only some will be utilized in a chemically useful way while some will be refluoresced or converted to heat. Since this involves time-light relationships, this factor will be covered after first considering the types of lamps that might be used. Comparing the three basic types of light sources (1) the high intensity discharge (HID) and (2) the fluorescent, and (3) the incandescent, with their efficiencies expressed in terms of the percent of electrical input energy that is transduced into visible spectrum light, these efficiencies are about 33%, 22% and 5%, respectively. It should be noted that these efficiencies are expressed in terms of visible spectrum and not in terms of utilization for photosynthetic purposes. In fact, fluorescent lamps are the only light source that can be tailored to provide near optimum absorption spectra and coupled with its controlability, cost and other factors, such lamps are the best lamp candidate in terms of utilization of the photon energy in a chemically useful way. To carry the discussion further, and assuming overhead lamp array with reflectors, about 50% of the photons generated will fly on intercept pathways with the chlorophyll molecules in the leaves. Further, it is estimated that of the light absorbed by the leaves, approximately twenty percent, takes the first singlet state pathway to chemical utilization in conventional systems. The remaining 80% is lost through fluorescence or continuing internal energy conversion. The HID lamp depends on direct arc radiation from a gaseous arc discharge hence the spectral energy cannot be independently controlled. Such lamps also operate at very high temperatures requiring the lamps to be located some distance from the plant. the first factor results in higher internal energy conversion in contrast to a red spectrum phosphor fluorescent energy and the second factor decreases the photon-chlorophyll intercept possibilities. The combined effect of these two factors offset the higher luminosity efficiency of the HID lamp. In fluorescent lamps relative narrow band light emitting phosphors can be used to provide a light output which is spectrally matched to the most efficient chlorophyll absorption band(s). Accordingly, the absorption potential of the light received from the fluorescent lamp is greater than that of the HID lamp. Moreover, by being of more optimum wavelength, a higher percentage of the light energy absorbed is utilized in a chemically useful way. When these factors together with the relative equipment costs are considered, the fluorescent lamp is generally favored over the HID type of lamp. Further, other reasons, yet to be discussed, clearly make the fluorescent lamp the superior choice.
To account for at least one factor that reduces the guantum efficiency a limited overview as to how the light absorption process physically works is also important. A representative photosynthetic unit of a leaf includes a population of about 300 light absorbing chlorophyll "B" molecules and a single special chlorophyll "A" molecule. Each plant might have millions of these discrete photosynthetic units with each working away as a tiny photosynthate factory manufacturing glucose independently of the others. Some of the light photons which fall on the units are absorbed by the chlorophyll "B" molecules, also referred to as antenna molecules. By a complex energy transfer mechanism, the trapped light energy is then transferred to the single special chlorophyll "A", or energy processing molecule. Up to this point the process is rapid, perhaps only a few nanoseconds, but once the "A" molecule is fueled the biochemistry begins with its related processing time taking upwards to a few milliseconds. During this processing time period no more energy can be taken in by the single chemical processing molecule. If the energy held by an electron in one or more of the chlorophyll molecules cannot be transferred for processing within the finite time the absorbing electron can stay excited, the energy must then be disposed of elsewhere. Therefore, this energy, if not utilized in a chemically useful way, is lost so far as the photosynthetic process is concerned. If all photons absorbed could be utilized in a chemically useful way, about three first singlet state energy equivalent photons would be required for each molecule of final form carbohydrate produced. Converted to energy equivalents, and multiplied by the number of molecules (Avogadro's constant) per mole, the product would be the 112 KCAL previously discussed (or 672 KCAL mole for glucose). We are dealing here with a theoretical 100% quantum efficiency in which the energy from each quanta of light would hypothectically be utilized in a chemically useful way. In nature, the quantum efficiency is a few percent at best. Most researchers contend that 8 photons under laboratory conditions are required for each carbohydrate molecule synthesized. On this basis, and depending upon the wavelengths involved, between 18 and 30% of the absorbed photon energy would be chemically utilized which reasonably corresponds to the 20% referred to earlier.
Turning again to a comparative consideration of light sources, a further very important advantage of fluorescent lamps over HID lamps is that the former can be rapidly pulsed on and off while the latter cannot. A plant growth system which utilizes pulsed fluorescent lamps is disclosed in my co-pending U.S. Pat. application Ser. No. 346,902, filed on Apr. 2, 1974, which issued as U.S. Pat. No. 3,876,907 on Apr. 8, 1975, and which is a divisional application based on my earlier filed U.S. Pat. application Ser. No. 96,789, now abandoned. In this system relatively short pulses of light are used, each followed by a longer dark period which is on the order of milliseconds long. During the latter no more light energy can be utilized until the processing of the energy transferred to the energy processing chlorophyll "A" molecule is completed. Thus, photons from pulsed fluorescent lamps can be delivered to a plant in a time related manner with a higher likelihood that the special chlorophyll "A" processing molecules is at the ground state and able to receive the photon energy, thereby providing a higher utilization efficiency in terms of photons received versus photons chemically utilized. Also, the size and related cost of the iron core and copper components of the lamp system are reduced because of the limited duty cycle. Although the use of a properly mechanized pulsing fluorescent lamp obviously provides a corresponding reduction in the amount of electrical energy required, the economics of using artificial light are still such that practical systems cannot complete with conventional plant growth systems using natural light for most applications.